Is 8-byte alignment for “double” type necessary?

Question

I understand word-alignment, which makes the cpu only need to read once when reading an integer into a register.

But is 8-byte alignment (let\'s assume 32bit system

Answer 1

• I seem to remember that the recommendation for 486 was to align double on 32 bits boundaries, so requiring 64 bits alignment is not mandatory.

• You seem to think that there is a relationship between the data bus width and the processor bitness. While it is often the case, you can find variation in both direction. For instance the Pentium was a 32-bit processor, but its data bus size was 64 bits.

• Caches offer something else which may explain the usefulness of having 64-bit alignment for 64-bit types. Here the external bus is not a factor, it is the cache line size which is important. Data crossing the line cache is costlier to access than data not crossing it (even if it is unaligned in both cases). Aligning types on their size makes it sure that they won't cross cache lines as long as cache line size is a multiple of the type size.